The present invention relates to a method of making a heat exchanger, in particular a motor vehicle radiator, and to a hollow shape for use in such a method.
Hitherto, radiators, in particular water radiators for internal-combustion engines of motor vehicles, have been made in Europe about 20% from conventional brass and copper tubes and about 80% from aluminum tubes.
Aluminum radiators, which due to their higher efficiency and their lower weight are preferred to the radiators of brass and copper tubes, are made in the form of plug-type radiators with plugged aluminum tubes or using welded aluminum tubes. The proportion of plug-type radiators of aluminum is about 50%, while the proportion of radiators of welded aluminum tubes is about 30%.
Radiators with welded aluminum tubes are in particular used in internal-combustion engines in which a high cooling power is required, as is the case in particular in engines for small trucks and high-performance engines for automobiles. The welded tubes for such radiators are made in an operation wherein metal sheets are rounded from strip metal and welded by pressure welding in the longitudinal direction at one side. These tubes have a flat profile, in contrast to the tubes for plug-type radiators which have a circular profile, and are coated on the outside with AlSi alloys. This silicon coating is applied during the rolling of the sheet metal itself prior to the rounding forming operation and serves to enable and welded tubes to be subsequently soldered to a water tank and cooling fins.
This soldering of the welded flat tubes, which consist of, e.g., an Al Mg Si 0.5 or an Al Mn alloy, to the water tank and the cooling fins of the same or similar material is made by brazing or hard soldering at a temperature above 450.degree. C., in particular at 607 to 636.degree. C., with the aid of flux or under vacuum without flux.
Both these possibilities of soldering the welded tubes to the fins and the water tank involve serious problems. When soldering with a flux the environmental problems are serious because detrimental substances are liberated by the flux and are very aggressive to aluminum and iron sheet and must therefore be washed off the soldered radiator to prevent the latter from corroding. Furthermore, resulting fumes must also be removed from the air to prevent sheet metal, possibly pressed in the vicinity for the vehicle bodyworks, from being attacked. For this purpose air cleaning, which involves high costs, is necessary. When soldering without flux these problems, due to detrimental substances, admittedly do not occur, but because this type of soldering must take place under vacuum and requires extremely accurate temperature control, high costs are also involved. Due to the high temperature of the soldering under vacuum, in addition only a short holding time in the soldering furnace is possible so that at the soldering joint a spot tends to be formed rather than an area contact, and this impairs the thermal conductivity at this point.
In radiators hitherto having such welded flat tubes, usually two tubes are arranged parallel to each other, each having a width of, for example, 22 mm, such width being the outer dimension of the tubes in the direction of the larger major axis of the flat profile. It has, however, been found that the cooling efficiency increases with increasing outer dimensions, so that preferably only one flat tube with a correspondingly larger width should be provided. In the manufacture of radiators, however, the flat tubes are laterally screwed and pressed to the fins before the soldering operation to obtain a continuous exactly defined area of contact between the two components, and thereafter are soldered in the furnace. Due to the pressure on the flat tube during pressing and the temperature existing in the furnace during soldering, however, a deformation (collapse) of the side faces of the flat profile occurs in the furnace because the aluminum becomes soft and sinks inwardly when the pressure is applied. This effect occurs in particular due to the fact that the aluminum tubes have a very small wall thickness of, for example, only 0.4 mm.
Furthermore, after the soldering of the tubes and the fins, the core of the radiator formed in this manner is fitted into the radiator housing or into a radiator bottom, and this operation must not cause any stress in the assembled radiator. However, due to the deformation of the components during soldering the necessary exact fit is difficult to achieve.
The problems regarding the collapse of the tubes and the lack of an exact fit become even more serious when, for increasing efficiency, wider flat profile tubes are used. Due to the aforementioned deformation of the tubes during soldering the waste in production then increases, and the silicon coating of the tubes leads to a further problem because these reject radiators cannot simply be scrapped.